1. Field of the Invention
The present invention relates to an electric power steering controller for automobiles which causes a motor to generate torque for assisting the steering torque of a driver to assist the steering force of a steering system.
2. Description of the Prior Art
FIG. 40 is a block diagram showing the constitution of an electric power steering controller of the prior art disclosed by Japanese Laid-open Patent Application No. 7-186994, for example. In the figure, reference numeral 1 denotes a steering torque detector for detecting steering torque when a driver controls the steering wheel, 2 a steering torque controller for computing an assist torque signal based on the output of the steering torque detector 1, 3 a damping compensator for computing a damping compensation signal based on the output of a motor speed detector 5, and 4 an inertia compensator for computing an inertia compensation signal based on the output of a motor acceleration detector 6. Denoted by 7 is a motor current decision unit for computing a target current signal from target torque which is the sum of an assist torque signal, damping compensation signal and inertia compensation signal, computed by a first adder 12, 8 a decision unit for judging whether the output of the steering torque detector 1 and the output of the motor speed detector 5 have the same direction and outputting its decision result to the steering torque controller 2, the damping compensator 3 and the inertia compensator 4, 9 a motor drive unit for determining voltage to be applied to a motor 10 based on a difference obtained by a second adder 13 between a target current signal and a motor current value detected by the motor current detector 11 and applying the above voltage to the motor 10, and 10 the motor for generating assist torque which is substantially proportional to a motor current value which changes according to the above applied voltage to drive a steering unit. Reference numeral 14 denotes a car speed detector for detecting car speed and outputting its detected car speed signal to the steering torque controller 2, damping compensator 3 and inertia compensator 4.
A description is subsequently given of the operation of the electric power steering controller of the prior art.
When the car driver controls the steering wheel, steering torque at this point is measured by the steering torque detector 1 and output to the steering torque controller 2. The steering torque controller 2 computes an assist torque signal which is substantially proportional to the output signal of the above steering torque detector 1 and based on which the motor 10 is driven to assist the steering torque of the driver so as to reduce it.
At this point, the decision unit 8 judges whether the output of the steering torque detector 1 and the output of the motor speed detector 5 are the same. When they are the same, the damping compensator 3 and the inertia compensator 4 are not activated, the steering torque controller 2 determines target torque based on the assist torque signal which is determined based on the output of the steering torque detector 1 and a car speed signal from the car speed detector 14, and the motor current decision unit 7 determines a motor drive current. When they are not the same, the steering torque controller 2 is not activated, target torque is determined based on the outputs of the damping compensator 3 and the inertia compensator 4, and the motor current decision unit 7 determines a motor drive current. When the car speed is low at this point, the direction of target torque is made the same as the revolution direction of the motor and when the car speed is high, the direction of target torque is made opposite to the revolution direction of the motor. Therefore, when the driver turns the steering wheel, the steering torque of the driver is assisted to lighten torque required for steering. When the driver returns the steering wheel, the motor 10 is controlled such that the steering wheel is assisted to return to its starting point when the car speed is low and prevented from returning to its starting point at an excessive rotation speed when the car speed is high.
Generally speaking, the driver controls the steering wheel when turning a curved portion of a road or an intersection and returns the steering wheel to its starting point, making use of the spontaneous return force of the steering wheel generated by the road reaction torque of tires when returning to straight-line driving. However, since the road reaction torque of the tires is small when the driver controls the steering wheel slightly at a low car speed or a high car speed, the road reaction torque becomes smaller than friction torque in the steering unit, whereby the steering wheel does not return to its starting point in many cases at the time of returning to straight-line driving. Therefore, in this case, the driver must add torque to the steering wheel to return it to its starting point, thereby deteriorating a steering feeling.
In contrast to this, in the prior art, it is judged whether the output of the steering torque detector 1 and the output of the motor acceleration detector 6 are the same or not when the car speed is low, and a motor drive current is determined such that the motor 10 revolves in the same direction as the revolution direction of the motor when they are different, thereby improving the returnability of the steering wheel at a low car speed.
However, in the above prior art, when the driver controls the steering wheel with the small road reaction torque of the tires, for example, when the driver turns an intersection at a low speed or a gently curved portion of a road at a high speed, the steering wheel is stopped unless torque is added in a direction for returning the steering wheel, whereby the motor 10 does not revolve. Since the decision unit 8 cannot judge whether the output of the steering torque detector 1 and the output of the motor speed detector 5 are the same or not, it cannot determine a motor drive current for revolving the motor 10 in the same direction as the revolution direction of the motor and cannot improve the returnability of the steering wheel.
Further, since a motor drive current can be determined only to revolve the motor in a direction opposite to the revolution direction of the motor at the time of high-speed driving in the above prior art, the returnability of the steering wheel cannot be improved.
It is an object of the present invention which has been made to solve the above problems to provide an electric power steering controller which enables the steering wheel to be returned to its starting point without adding torque in a direction in which the driver returns the steering wheel when he/she controls the steering wheel with the small road reaction torque of the tires, for example, when he/she turns an intersection at a low speed or when he/she turns a gently curved portion of a road at a high speed.
According to a first aspect of the present invention, there is provided an electric power steering controller which comprises road reaction torque detection means for detecting road reaction torque received by tires from a road and road reaction torque addition control means for controlling the torque of a motor for assisting the steering torque of a driver based on road reaction torque detected by the road reaction torque detection means.
According to a second aspect of the present invention, there is provided an electric power steering controller which further comprises steering torque detection means for detecting the steering torque of the driver and steering assist control means for controlling the torque of the motor based on the output of the steering torque detection means.
According to a third aspect of the present invention, there is provided an electric power steering controller wherein the amount of control of the above road reaction torque addition control means is a value obtained by multiplying the output of the road reaction torque detection means by a single gain or multiple gains and the maximum value of control is limited by a limiter.
According to a fourth aspect of the present invention, there is provided an electric power steering controller wherein car speed detection means for detecting car speed is provided and the values of the above gain and limiter are set according to the output of this car speed detection means.
According to a fifth aspect of the present invention, there is provided an electric power steering controller wherein the values of the above gain and limiter are set such that the output of the road reaction torque addition control means becomes large when the car speed is low excluding the case where the car speed is extremely low at the time of parking.
According to a sixth aspect of the present invention, there is provided an electric power steering controller wherein motor current detection means for detecting a current running through the motor and motor rotation angle acceleration detection means for detecting the rotation angle acceleration of the motor are provided, and the road reaction torque detection means obtains a road reaction torque detection value by passing through a low-pass filter a value obtained by subtracting motor inertia torque in terms of a steering shaft computed from the output of the above rotation angle acceleration detection means from the sum of the output of the steering torque detection means and motor torque in terms of the steering shaft computed from the output of the above motor current detection means.
According to a seventh aspect of the present invention, there is provided an electric power steering controller wherein motor current detection means for detecting a current running through the motor is provided, and the road reaction torque detection means obtains a road reaction torque detection value by passing through a low-pass filter a value obtained by adding up the output of the steering torque detection means and motor torque in terms of the steering shaft computed from the output of the above motor current detection means.
According to an eighth aspect of the present invention, there is provided an electric power steering controller wherein motor rotation angle acceleration detection means for detecting the rotation angle acceleration of the motor is provided, and the road reaction toque detection means obtains a road reaction torque detection value by passing through a low-pass filter a value obtained by subtracting motor inertia torque in terms of the steering shaft computed from the output of the above motor rotation angle acceleration detection means from the output of the steering torque detection means.
According to a ninth aspect of the present invention, there is provided an electric power steering controller wherein motor torque detection means for detecting torque generated by the motor and motor rotation angle acceleration detection means for detecting the rotation angle acceleration of the motor are provided, and the road reaction torque detection means obtains a road reaction torque detection value by passing through a low-pass filter a value obtained by subtracting motor inertia torque in terms of the steering shaft computed from the output of the above motor rotation angle acceleration detection means from the sum of the output of the steering torque detection means and the output of the above motor torque detection means.
According to a tenth aspect of the present invention, there is provided an electric power steering controller wherein motor rotation angle acceleration detection means for detecting the rotation angle acceleration of the motor is provided, and the road reaction torque detection means obtains a road reaction torque detection value by passing through a low-pass filter a value obtained by subtracting motor inertia torque in terms of the steering shaft computed from the output of the above motor rotation angle acceleration detection means from the sum of the output of the steering torque detection means and the target value of motor torque in terms of the steering shaft computed from the target value of current for driving the motor.
According to an eleventh aspect of the present invention, there is provided an electric power steering controller wherein motor current detection means for detecting a current running through the motor is provided, and the road reaction torque detection means obtains a road reaction torque detection value by passing through a low-pass filter a motor torque value in the terms of the steering shaft computed from the output of the above motor current detection means.
According to a twelfth aspect of the present invention, there is provided an electric power steering controller wherein motor current detection means for detecting a current running through the motor and power voltage detection means for detecting power voltage applied to the motor are provided, and the rotation angle acceleration detection means computes the rotation angle acceleration of the motor from the output of the above motor current detection means and the product of the output of the power voltage detection means and the duty ratio of a PWM signal for driving the motor.
According to a thirteenth aspect of the present invention, there is provided an electric power steering controller wherein the road reaction torque detection means obtains the output of the steering torque detection means through a low-pass filter.
According to a fourteenth aspect of the present invention, there is provided an electric power steering controller wherein the break frequency of the above low-pass filter is set to a frequency band at which the driver generally controls the steering wheel and the influence of the friction torque of the steering unit upon a detection error which affects the road reaction torque detection value is minimized.
According to a fifteenth aspect of the present invention, there is provided an electric power steering controller wherein the break frequency of the above low-pass filter of the road reaction torque detection means is set according to the output of the car speed detection means.
According to a sixteenth aspect of the present invention, there is provided an electric power steering controller wherein the output of the steering torque detection means is applied to the road reaction torque detection means through the low-pass filter.
According to a seventeenth aspect of the present invention, there is provided an electric power steering controller wherein the output of the steering torque detection means which is applied to the road reaction torque detection means of the sixth aspect has a dead zone at around xe2x80x9c0xe2x80x9d of the steering torque signal.
According to an eighteenth aspect of the present invention, there is provided an electric power steering controller wherein the output of the road reaction torque detection means of the first aspect has a dead zone at around xe2x80x9c0xe2x80x9d of the road reaction torque.
According to a nineteenth aspect of the present invention, there is provided an electric power steering controller wherein the road reaction torque addition control means comprises distortion measuring means in a rack.
According to a twentieth aspect of the present invention, there is provided a method of controlling an electric power steering controller which comprises the steps of detecting road reaction torque received by the tires from the surface of a road and controlling the motor for generating torque for assisting the steering force of the steering system based on this road reaction torque.
The above and other objects, features and advantages of the invention will become more apparent from the following description when taken in conjunction with the accompanying drawings.